Magnetically focused electron gun



Aug. 26, 1952 Filed' June 17, 1950 M. E. HINES MAGNETICALLY IFOCUSED ELECTRON GUN 2 Mfrs- Smm 1 /NVE/v To@ -M L'. H/NES ATTORNEY Allg- 26 1952 M. E. HINEs 2,608,668

MAGNETICALLY FocusEn ELECTRON GUN Filed June 17, 1950 2 SHEETS-SHEET 2 REQU/RED MAGNET/C F/ELD PATTERN F/G, 2 50u/Po TENT/AL: ARE CoA/FOCAL PARABOL o/Ds 0F REvoLur/ON EQU/PO TEN T/A L5 FLUX L//vEs l' BEAM CONVIBQG'IRS- TOWARD COMMON FOCUS ELECTROSTAT/C F/ELD DUE TO SPACE CHARGE E OU/POZ'EN T/ALS ARE CONES OFCONSTANT 6 M. E. H/NES BV ATTORNEY Patented Aug. 26, 1952 l UNITED.N t STA-TES PATENT; OFFICE A w`-z,eos,6es f MAGNETICALLYFOCUSED ELECTRDN GUN E f Marion E. Hines, Summit, N. J., assignor to B'ell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application June 17, 195o, serial No. 168,303

` (c1. els- 4) This invention relates to electron discharge devices and, more particularly, to electron guns forhigh current electron V,beam devices,

In general, in a number of `known general types of electron beam dischargedevices, high current beams are imminently desirable. In some devices, the desired high current may be realized by utilization of beams of substantial diameter. In others, however, forV example, in vhigh frequency devices of the traveling wave type, a requisite for optimum operation is that the beam be of very small diameter and, further, that the diameter remain substantially constant over a long path. Ways of maintaining the beam di-x ameter small and substantially constant between the inlet and outlet ends of a given region have been suggested in the art. However, there still remains the problem of obtaining a beam of the requisite or desiredV high density at the point of injection into the given region.

Known electron guns involving conventional focusing methods suffer a seriouslimitation due to space charge effects. Specifically, forces resulting from space charge causeya' divergence of the beam and the greater the current density, the greater these forces,` and the greaterthe tendencytoward beam divergence.y Thus, in known devices in general, a choice or compromise must be made between current density in the `beam and beam diameter.

One general object of this invention is to enable realization of high current densitiesin an electron beam concomitantly with substantially minimum beam diameter.` More specifically, one object of this inventionis to enable attainment of'maximum current density in a beam of given diameter, or. viewed in another way, to attain aminimum beam diameter for agiven current density. t

'Another object of this invention is to provide a magnetically focused electron gun for produc ing a small diameter beam, `which comprises a large area'cathode and, consequently, islcapable of projecting a high current,` intense electron beam.

A further object of this invention is to nullify space charge repulsion effects in an electron beam.

Still another Objectis to improve electron guns generally. t

In accordance with a feature of this invention an electron beam of circular' cross section is projected through a circular,l aperture in one element of a magnetic system, which `creates a radial .magnetic field` in the,V- region: of `said aperture having'a `magneticfleld component thereof perpendicularftov thev fiow. of saidV beam; ,i This field fcomponent:` is i greatest` atf the; peripheryr Of said aperture and is `zeroat 4the'icenter of said aperture. The electronbeam-isjfthus` given a y 13 Claims.

rotating motion. The magnetic lines of force are then converged in a substantially' parabolic manner. The rotating electron beam, which has been focused to form a converging conicalshaped beam having the same axis as the magnetic eld, is held withinlthat shape by the force of its interaction with the' converging magnetic field, which is just great enough to supply the radial accelerating force ofthe rotating beam and to nullify the 'space charge effects in the electron beam.

A feature of the invention pertains to a converging conical-shaped rotating electron beam in conjunction with a converging paraboloidal magnetic field, which permits the beam from a large cathode to be focused to a smaller diameter that has heretofore been possible.

The above-mentioned and otherfobjects and features of the invention will be more clearly understood from the following detailed description when read in conjunction with the drawings, in which: Y

Fig. l is a view, mainly in section, of a discharge device including an electron gun illustrative of one embodiment of the invention;

Fig. 2 is a diagram illustrating the magnetic field in the gun shown in Fig. 1;

Fig. 3 is a diagram illustrating the electrostatic eld due to space charge; and

Fig. 4 is another diagram showing the various components' of velocityof the electron beam;

Referringnow to Fig. 1, the cathode I I is heated by heater coil I2, which is energized by battery 5. The electron beam represented by dotted lines I3 is focused and accelerated by` electrodes I4 and I5, the geometry of `electrodes I4 and I5 being such that the electron beam I3is of con* ical shape. Electrode `I 5 is maintained atground potential, and electrode I4 is maintained at approximately -1500 volts by means of Ibattery 6. Reference .is vmade toV United States Patent 2,268,197, issued December 30, 194:1,` to J. R. Pierce, wherein `a similar geometry is discussed in detail. Electrode I5 is of a magnetic material having an aperture I 6 therein through which the electron beam passes. i l A A The magnetic field established between polepieces I5 and I'I, as described hereinaftenhas a radial component thereof perpendicular tothe axis of said apertureflli'which causes theelectron beam passing through toacquire an angular velocity. The magnetic lines of4 force between pole-pieces and I'I have a paraboloidal shape having thesame axis as theaxisof the electronl beam. The two pole-pieces l5 and `I1 are separated and supportedby a Enon-magnetic shield I8, and thet-pole-p/iece I'Ifis .magnetically positive with` respect tothe pole-piece I5. `As the electron beam `iiows from aperture vI6 to aperture I9. there is an interaction `between the aeoacee a cylindrical element 3|. The helix 25 is supportedby rods 32.` L

Coil 33 generates the fiux which flows in a path determined by magnetic element 34 across f an air-gap to magnetic element forming a V' converging paraboloidal field inside non-magnetic element I8, through magnetic element Il,

tron volt energy of the electron stream, and the angular velocity of said rotating electron stream. These parameters will be defined more clearly by detailed analysis later herein.

' It is noted that surfaces 40 and 4| of elements 34 and l5 comprise a paraboloidal surface except where envelope I0 separates elements 34 and I5.

' Simi1arly,'surfaces`42 and '43 of elements 2| and I1 also; form a paraboloidal surface. The focal points of these two p'araboloidal surfaces are substantially coincident with the focal point of the conical-shaped beam, which would theoretically roccur 'in the vicinity of aperture I9. rhese surfaces represent equipotential surfaces of the magnetic eld existing therebetween. The flux generated between these two surfaces lthen will also have a confocal paraboloidal geometry with the focal point substantially coincident with the focal point of the converging electron beam. This eld is represented in Fig. 2, wherein the equipotentials and the `flux lines at a plane passing through the axis of the electron beam are shown. It is not essential that the surfaces 4B,

4| and 42, 43 be perfect confocal paraboloids A of revolution, since other curved surfaces will also produce similar magnetic flux eldswhich can be suitably utilized in obtaining a converging electron beam. However, in the preferred embodiment of the invention described herein, paraboloidal surfaces are used.

The intensity of the magnetic iield increases as the beam becomes 'smaller in diametenalways, however, being of such intensity as to provide interaction forces sufficient to lcancel the increased space charge forces of the electron beam and the increased centrifugal forces due to the rotational motion of the electron beam. This will beco'me clearer from the mathematical analysis which will be presented hereinafter.

In thef'aperture I9 in the electrode il, the magnetic field is weaker Ythan it is within the non-magnetic element I8, thus allowing the electron beam to diverge until the electron beam flow becomes cylindrical in shape. Upon leaving the aperture I9, the electron beam, which is now uniformly cylindrical, enters a magnetic eld which is substantially parallel to the fiow of the electron beam and 'of sufcient strength to maintain the uniformly 'cylindrically-shaped electron beam as it travels through the helix 25. Electrode 28 constitutes the collector for the electron stream and is maintained at approximately -1000 volts. In this particular embodiment of the invention, the electron gun has been adapted to a traveling-wave tube of the type disclosed inthe application of L. M. Field, Serial No. 704,918, filed October 22, 1946, now Patent No. 2,575,383, issued November 20, 1951.

The signal to be amplified is injected by means of wave guide 22 upon antenna 23, which is secured to cylindrical element 24 and to the helix 25,*which is grounded through lead 29. The signal travels along the helix with approximately the same forward component of velocity as of the electron stream Yand is fed into wave guide 21 by `means of antenna 26, which'is secured to through magnetic. elements 2|, and back to ele- 'rn'ent 34. It is to be noted that magnetic elds havingradial components are formed in apertures I6 and I9. Coil 35 generates a flux which flows through a path comprised of magnetic electrode 2|, magnetic electrode I1, forming a magnetic 'ileld substantially parallel to the flow of the electron beam from element 24 to element 3|, and back to `magnetic material 35.

A fuller 'understanding of the operationof this invention may be obtained V'from the following mathematical analysis. vDue"to the particular type of magnetic and electrostaticelds V-p'resented in Athis invention, spherical polar coordinates will be used to describe the electron 'paths and the electric and magnetic fields.,

The orientation of the coordinates jis `shown in Fig. 4. MKS practical units will be used. Some of the symbols used are den-ned below.

q charge density expressed in .coulombs .per unit solid angle per meter along the radius.

p charge density in coulombs per cubic meter.

n charge to mass ratio of the electron, 1..`*i6 1.0u

Vcoulombs per kilogram.

e dielectric constant of vacuum,

farads per meter.

r radial ccordinateiused inthe spherical polar coordinate system. A

0 angular displacement ofrvfroma reference line which is hereinV chosen Ito coincide 'with the axis of the electron beam.

angular displacement lof '1' from `a kreference plane. The dot notation is Suse-d to 'indicate total time derivatives, 1- indieating VAcir/dt, l'and r indicating 12T/dt?, etc. Y

It is to be noted the phrase Affifalial component when used in connection with the .mathematical analysis applies to the spherical .polar coordinate system utilized herein.

The method of solution -is indirect, -in that an assumed set of conditions --is shown to satisfy the equations of motion for small -angles 4ofconvergence. The first assumptions are that the radial velocity is constant and Aindependent of 0 and that there are v no motions in the .0 direction. A magnetic viield,coniigurat-ior-i fis chosen whose radial component is inversely .proportional to the radius as shownin .Fig/s. A2 and 4. Such a field is described by the following equations:

where B1 is the magnitude of the 'lr'adial component ofthe fleld'atunity radius, Bffis the :flux in theirdirection,Be i`s thefluxli'nthe 29 direction, and Bc is thethe direction. This field is a solution of Laplaces equation.V The magnetic -fiux'lin'es and-jthe magnetic ecuiipotential surfaces conform to thesurfaceslof paraboloids of revolution, which have 'afcommon :focus at the origin of coordinates.

t is also assumed that the cathode is shielded from the` magnetic field and that all fleldsare axially symmetric. From vBuschs theorem. the

angular velocity can be deduced for electrons at any point in the space. The theorem specifies that n@ 2r2 sin2 0 2r cos2 g The velocity `distribution of the electrons is further Vspecified by a constant r velocity, and zero 0 velocity, as summarized below.

If it is also assumed that a uniform radial current density exists over the cone, the electrostatic eld coniiguration due to space charge can be found. For a conical conducting boundary such as element I8 shown in Fig. 1, the field within the beam is given by tan (8) where q is the charge density expressed in cou-` lombs per unitsolid angle per unit radius. This is 4a solution to Poissons equation.` The electrostatic equipotentials are cones Whose apexes are at the origin of the coordinates. The' electrostatic flux lines conform to the surfaces of spheres whose centers are at the origin of coordinates. This is shown in Fig. 3. The total current I in amperes carried Within a cone bounded by an angle i/f is i I=41rqy/21,V0 sin2 amperes l0) There is now established a set of assumed conditions specifying the velocity distribution and the static iields. It remains to demonstrate that these conditions satisfy the equations of electron motion. These equations are Substitution ofthe assumed values for the velocities, e1ds,` andacceleratons into these equations Equation 12 reduces -1I2B12 tan COS 9 -nq tan g 112B12 tan g `2f Lew; e.' 7,.

(12) Another substitution eB 2 q:-- 2 1 14) yields 7721312 tan cos 0 N 7121312 tan g 2r 4I COS2 d 21 2 This approaches an identity for small 0 where the expression in the brackets may be considered equal to 1. It is very nearly an identity even for 0 as large as l0 degrees where the ratio of cosines is .992. This discrepancy represents an unbalance of forces in the 0 direction which is practically negligible for reasonably small 0 on the order of 0,-10 degrees. This means that the assumed velocity distribution is not quite correct for the outer electrons in a wide angle beam, but the discrepancy is negligible for reasonably small cone angles.

The substitution made above speciiied by Equation 14 was necessary to make Equation 12 becomean identity. .It species the magnetic eld B1 required for any degree of space charge q. In accordance with this relation, the beam current within a bounding angle which is the maximum valueof 0, may be expressed as 'I= zx/frenS/BIZI/w sin g 15) This isfthe relation which relates beam current I, beam voltage Vo, cone angle 1p, `and the magnetic eld at unityradius B1. This is the basic design equation for the region of conical electron iiow. In MKS units, I is expressed in amperes, Vo in volts, and B1 in Webers` per square meter. It should` be noted that vB1 is the `magnetic eld Whicnwould exist at a radius of 1 meter from the origin of the coordinate system, and the radial component of magnetic field at any other radius is equal to Bi/r.)4 It is to be further noted that Vo is the potential at the beam axis and will ordinarily be slightly more negative than the potential oi' the shield electrode.

The potential V inthe beam is a function of the angle 0 only and is given by :VIVI-2 5 log cos 9 0 tan 2- -1s approximately equal to 2 `Equavtion may be approximated asA If the electrons were emittedfrom a cathode at. zero potential, their kinetic energies must correspond to the space potential V, as expressed by the equation v t which reduces to an identity for small if which is the conditionlrequired for satisfying the equations of motion.. Thus, the type of electron flow necessary to realize this invention can be obtained from a unipotential cathode.

For the electrostatic eld, .we have postulated a cone-shaped conducting shell surrounding the beam. While this is necessary to obtain the simple iield described, in an actual case, it will probably not make a great deal of difference if it is shielded by a cylindrical or other shaped electrode. For the cone-shaped boundary, the electrostatic elds will be oriented as shown in Fig. 3, wherein the equipotential surfaces are represented by the radial lines from the origin and the electrostatic field is represented by the arcuate lines.

The magnetic field conguration is shown in Fig. 2. Both equipotentials and nur; lines conform to the surfaces of paraboloids of revolution, which have a commonfocus with the electron stream. The required field may be realized with paraboloidal pole-pieces of large size and may ce apprcximatedclosely with smaller pole-pieces of other shapes. These shapes can be experimentally determined by the use of an electrolytic tank. Y

[is discussedbefore, Fig. l shows a configura tion of electrodes which produces this type of iiow. At the left, there is a converging gun of generally conventional construction. The accelerating anode of this gun is made of a magnetic material of high permeability and is shaped to shield the internal parts of the gun from the magnetic field. As the beam passes through this anode, itrenters the magnetic eld rather suddenly, which gives the beam a twist, providing the initial l velocity. The total input velocity determines the cone angle and fixes the origin of the coordinates used to describe the motion and the magnetic iield. The origin is chosen so that the electrons do not have any velocity in the H direction after they have entered the magnetic eld.

It is to be understood that the specific embodiment of the invention herewith shown and described is to be taken as a preferred example of the same and that various changes in shape, size, and arrangement of parts may be resorted to and other applications may be made thereof Without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge device comprising a first and a second spaced magnetic member having axially aligned apertures therein, means for producing a conically converging electron beam, means for projecting said beam through said apertures, andmeans includingsad first magnetic membenand magnetic means for producing avmagnetic neld configuration betweensaid spacedmembers whose. magnitude in the direction along the axis isinversely proportional to the axial distance from the point toward which the beam converges.

2. An electron discharge device comprising a magnetic element Vhaving an aperture therein, meansfor projecting an electron beam through said aperturein the direction of the axis of said aperture;l magnetic means coupled to said element for producing at said aperture a magnetic eld having a radial component increasing from said axis, and means for focusing said beam at a preassigned region beyond said aperture, said last means comprising means for producing a paraboloidal magnetic field beyond .said aperture and including said magnetic element and said magm netic means.

3. An electron discharge device in accordance` with claim 2, wherein said focusing means also comprisesl al second magnetic element havinga second aperture thereinand positioned near the focalV point of said electron beam, said second aperture having the same axis as said electron beam, said second magnetic element forming a return path for a portion of the flux from said magnetic means.

4. An electron discharge device in accordance with claim 3, wherein a second magnetic means is positioned so as to produce a substantially parallel magnetic field in the region of and substantially parallel to the flow or" said electron beam 'after said electron beam leaves Saidsecond aperture, said substantially parallel magnetic eld being of strength to maintain said electron beam in its cylindrical' path.

5. An electron discharge device comprising means for producing a converging electron beam, a'rst magnetic element having a first substantially circular aperture therein, said iirst magnetic element having a substantially paraboloidal surface with a focal point substantially the same as the focal point of said electron beam, a second magnetic element having a second substantially circular aperture therein, said second magnetic element having a substantially paraboloidal surface with substantially the same focal point as the focal point of said electron beam, the ksaid apertures having their axes coincide with the axis of said electron beam, second means fo-r creating a Vradial magnetic iield in `said first aperture to cause said electron beam to rotate as ity passes therethrough, said second means for creating a magnetic eld further creating aV magnetic eld of substantially confocal paraboloids of revolution with the focal point substantially the same as the focal point of said electron beam. v

6. AnV electron discharge device in accordance with claim 5, wherein said second magnetic element provides a return path for a portion of the magnetic iiux between said first and second aper tures.-

7. An electron discharge device in accordance with claim 6, wherein a third means for creating a magnetic held is positioned around a portion of the'path of said electron beam so as to produce a substantially parallel magnetic eld in the region ofan substantially parallel to the flow of said electron beam after said'electron beam leaves said second aperture, said substantially parallel magnetic field being of strength to maintain said electron beam in its cylindrical path; Y

8. An electron discharge device comprising an evacuated envelope, a transmission path within said envelope capable of guiding high frequency electrical Waves, said transmission path comprising a conductor in the form of an elongated helix, a first means to impress waves to be amplified upon an input end of said transmission path to permit travel of the waves along said path, a second means to couple a load circuit to an output end of said transmission path, a third means to produce and accelerate an electron stream through said helix, said third means comprising a fourth means for generating and accelerating an electron beam, a first magnetic means comprising an element having an aperture therein for creating a magnetic field which has a radial component perpendicular to the axis of said aperture, said electron beam passing through said aperture With its axis substantially parallel to the axis of said aperture to give said electron beam a rotating motion, a second element having a second aperture therein placed at a distance from said iirst element, said electron beam passing through said second aperture, the adjacent surfaces of said rst and second elements being confocal paraboloids of revolution with a focal point coincident with the focal point of the electron beam, said iirst magnetic means creating a magnetic iield of confocal paraboloids of revolution between the said adjacent surfaces of said rst and second elements, said paraboloidal magnetic, field being of sufficient strength to supply radial accelerating force of the rotating electron beam and to counterbalance the space charge effect of the electron beam.

9. An electron discharge device in accordance with claim 8, wherein said second element is of a magnetic material and provides a return path for a portion of the magnetic flux between said iirst and second apertures, thus lessening the intensity of the magnetic eld in said second aperture and causing said converging electron beam to adapt a cylindrical path of flow.

10. An electron discharge device in accordancel with claim 9, wherein a second magnetic means for creating a magnetic field is positioned around 4 a portion of the path of said electron beam so as to produce a substantially parallel magnetic eld in the region of and substantially parallel to the flow of said electron beam after said electron beam lealves said second aperture, said substantially parallel magnetic eld being of sufiicient strength to maintain said electron beam in its cylindrical path.

11. An electron gun comprising a pair of spaced magnetic members having axially aligned apertures therein, the facing surfaces of said members being paraboloidal and respectively convex and concave, means for magnetizing said members to establish a magnetic eld therebetween, and means for projecting an electron beam through said apertures in the direction from said concave surface toward said convex surface, said beam projecting means comprising a cathode and electrode means thereadjacent for directing electrons emanating from said cathode along paths converging in said direction.

12. An electron discharge device comprising a first and a second magnetic member having axially aligned apertures therein, the surface of said first member facing said second member being substantially paraboloidal, means for projecting an electron beam through said apertures along the axis thereof, and means for producing a magnetic eld between said rst and second magnetic members.

13. An electron discharge device comprising first and second spaced magnetic members having axially aligned apertures therein, the surface of the rst of said membersfacing said second member being substantially paraboloidal, means for producing a converging electron beam, means for projecting said beam through said apertures along the axis thereof, and means for producing a magnetic field between said spaced magnetic members.

MARION E. HINES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,149,101 Ploke Febr. 28, 1939 2,212,206 Holst et al Aug. 20, 1940 2,258,149 Schutze Oct. 7, 1941 2,407,906 Rose -s Sept. 1'?, 1946 

